The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device in which viewing angle dependence of the display screen is improved by combining a liquid crystal display element with an optical retardation plate.
In the past, liquid crystal display devices using a nematic liquid crystal display element were widely used in numeral segment display devices in watches, calculators, etc., but recently, they have come to be used in devices such as word processors, laptop personal computers, and in-car liquid crystal televisions.
Generally, liquid crystal display elements include a transparent substrate, on which are provided electrode lines, etc. for turning the pixels on and off. For example, in an active matrix type liquid crystal display device, in addition to the electrode lines, active elements such as thin film transistors are provided on the substrate, as switching means for selective driving of pixel electrodes which apply a voltage to the liquid crystal. Further, in liquid crystal display devices capable of color display, color filter layers of, for example, red, green, and blue, are also provided on the substrate.
With regard to liquid crystal display methods used in liquid crystal display elements of this type, different methods are selected, as needed, depending on the twist angle of the liquid crystal. Two well-known examples are the twisted nematic liquid crystal display method (hereinafter referred to as the xe2x80x9cTN methodxe2x80x9d), which is an active driving method, and the super twisted nematic liquid crystal display method (hereinafter referred to as the xe2x80x9cSTN methodxe2x80x9d), which is a multiplexing driving method.
In the TN method, nematic liquid crystal molecules are aligned in an arrangement twisted 90xc2x0, and display is performed by guiding light along the direction of twist. The STN method, on the other hand, makes use of the fact that, by expanding the twist angle of the liquid crystal molecules to more than 90xc2x0, light transmittance changes abruptly around the liquid crystal""s applied voltage threshold.
Since the STN method makes use of the birefringence effect of the liquid crystal, color interference causes the background of the display screen to have a distinctive color. To remedy this kind of shortcoming and perform black-and-white display using the STN method, it is considered effective to use an optical compensation plate. Display methods which use an optical compensation plate can be broadly divided into the double-layered super twisted nematic phase compensation method (hereinafter referred to as the xe2x80x9cD-STN methodxe2x80x9d) and the film-type phase compensation method (hereinafter referred to as the xe2x80x9cfilm provision methodxe2x80x9d), in which a film having optical anisotropy is provided.
The D-STN method uses a two-layer structure of a liquid crystal cell for display and another liquid crystal cell in a twisted alignment with a twist angle of a direction opposite that of the liquid crystal cell for display. The film provision method, on the other hand, uses a structure provided with a film having optical anisotropy. From the point of view of weight and cost, the film provision method is considered superior. Since black-and-white display characteristics have been improved by adopting phase compensation methods of this type, color STN liquid crystal display devices have also been realized, which enable color display by providing a color filter layer in a display device using the STN method.
The TN method, on the other hand, can be broadly divided into normally-black and normally-white methods. In the normally-black method, two polarization plates are arranged so that their directions of polarization are mutually parallel, and black is displayed when an ON voltage is not applied to the liquid crystal (OFF state). In the normally-white method, two polarization plates are arranged so that their directions of polarization are mutually perpendicular, and white is displayed in the OFF state. From the points of view of display contrast, color repeatability, dependence of display on viewing angle, etc., the normally-white method is superior.
However, with TN liquid crystal display devices, since the liquid crystal molecules have anisotropy of the refractive index xcex94n, and since the liquid crystal molecules are aligned so as to tilt with respect to the upper and lower substrates, the contrast of the display image varies according to the direction and angle from which it is viewed; thus viewing angle dependence is large.
FIG. 12 schematically shows the structure of a TN liquid crystal display element 31 in cross-section. The Figure shows a state in which a voltage for display of a gray shade is being applied, and the liquid crystal molecules 32 are tilted up slightly. In the TN liquid crystal display element 31, a linearly polarized light ray 35, traveling in the normal direction of the surfaces of substrates 33 and 34, and linear polarized light rays 36 and 37 traveling at an incline with respect to the normal angle, cross the liquid crystal molecules 32 at different respective angles. Since the liquid crystal molecules 32 have anisotropy of the refractive index xcex94n, the linearly polarized light rays 35, 36, and 37 of each direction, in passing among the liquid crystal molecules 32, produce ordinary light and extraordinary light, and due to a difference in phase thereof, the linearly polarized light rays 35, 36, and 37 are converted into elliptically polarized light. This is the cause of viewing angle dependence.
Further, in an actual liquid crystal layer, the angle of tilt of liquid crystal molecules 32 around the midpoint between the substrates 33 and 34 differs from that of the liquid crystal molecules in the vicinity of either substrate 33 or 34, and the liquid crystal molecules 32 are also twisted 90xc2x0 along the axis of the normal direction.
For these reasons, in passing among the liquid crystal molecules 32, the linearly polarized light rays 35, 36, and 37 are subject to various birefringence effects depending on their direction and angle of travel, and show a complex viewing angle dependence.
Specifically, viewing angle dependence is evident as phenomena such as the following. As viewing angle is gradually inclined in the standard viewing angle direction (below the normal direction of the display screen), above a certain angle from the normal direction, phenomena occur such as coloring of the display image (hereinafter referred to as xe2x80x9ccoloring phenomenonxe2x80x9d) and reversal of black and white (hereinafter referred to as xe2x80x9creversal phenomenonxe2x80x9d). Again, as viewing angle is gradually inclined in the opposite viewing angle direction (above the normal direction of the display screen), contrast is drastically impaired.
Further, the foregoing liquid crystal display device has the problem that, as the size of the display screen is increased, the viewing angle field is decreased. If a large liquid crystal display screen is viewed at close range from directly in front of the screen, there are cases in which viewing angle dependence causes colors in the upper and lower parts of the display screen to appear to differ. This is because the actual angles of view needed to view peripheral portions of the screen are increased, which is equivalent to viewing the screen from a more inclined viewing angle.
In order to improve this kind of viewing angle dependence, Japanese Unexamined Patent Publication Nos. 55-600/1980 (Tokukaisho 55-600) and 56-97318/1981 (Tokukaisho 56-97318), for example, have proposed insertion of an optical retardation plate (optical retardation film), as an optical element having optical anisotropy, between a liquid crystal display element and one of the polarizing plates.
In this method, light which has been converted from linearly polarized to elliptically polarized light by passing among the liquid crystal molecules (which have anisotropy of the refractive index) passes through an optical retardation plate. Consequently, the difference in phase between ordinary light and extraordinary light which arises depending on viewing angle is compensated, and the elliptically polarized light is re-converted into linearly polarized light. Thus viewing angle dependence is improved.
Japanese Unexamined Patent Publication No. 5-313159/1993 (Tokukaihei 5-313159) discloses an example of this kind of optical retardation plate, in which one of the principal refractive index directions of the index ellipsoid is parallel to the normal direction of the surface of the optical retardation plate. However, use of this optical retardation plate results in only limited improvement of reversal phenomenon in the standard viewing angle direction.
For this reason, U.S. Pat. No. 5,506,706 (corresponding to Japanese Unexamined Patent Publication No. 6-75116/1994 (Tokukaihei 6-75116)) proposes a method which uses an optical retardation plate in which one of the principal refractive index directions of the index ellipsoid inclines with respect to the normal direction of the surface of the optical retardation plate. In this method, one of the following two types of optical retardation plate is used.
One of these is an optical retardation plate in which, of the three principal refractive indices of the index ellipsoid, the direction of the smallest principal refractive index is parallel to the surface of the plate, the direction of one of the two remaining principal refractive indices inclines at an angle of xcex8 with respect to the normal direction of the surface of the plate, and the direction of the other principal refractive index also inclines at an angle of xcex8 with respect to the normal direction of the surface of the plate. Here, the value of xcex8 satisfies the relation 20xc2x0xe2x89xa6xcex8xe2x89xa670xc2x0.
The other type of optical retardation plate is one in which the three principal refractive indices na, nb, and nc of the index ellipsoid satisfy the relation na=nc greater than nb, and the direction of the principal refractive index nb, which is parallel to the normal direction of the surface of the plate, and the direction of the principal refractive index nc or na, which is in-plane with respect to the surface of the plate, incline in a clockwise or counter-clockwise direction with respect to an axis which is the direction of the principal refractive index na or nc. In other words, the index ellipsoid inclines.
With respect to the first of the foregoing two types of optical retardation plate, either a uniaxial or biaxial plate may be used. Again, with the second type, a single plate may be used alone, or two such plates may be combined, with the respective principal refractive indices nb thereof inclining 90xc2x0 with respect to one another.
In a liquid crystal display device structured so that at least one such optical retardation plate with an inclined index ellipsoid is provided between a liquid crystal display element and a polarizing plate, change in contrast, coloring phenomenon, and reversal phenomenon, which occur depending on viewing angle, can be improved over an optical retardation plate in which a principal refractive index direction of the index ellipsoid does not incline with respect to the normal direction of the surface of the plate.
In addition, various techniques for resolving reversal phenomenon have been proposed, such as the so-called pixel division method disclosed in Japanese Unexamined Patent Publication No. 57-186735/1982 (Tokukaisho 57-186735), in which each display pattern (pixel) is divided into a plurality of portions, and alignment control is performed so that each portion has independent viewing angle characteristics. With this method, since the liquid crystal molecules tilt up in a different direction in each portion, viewing angle dependence when viewing angle is inclined upwards or downwards can be improved.
Further, Japanese Unexamined Patent Publication Nos. 6-118406/1994 (Tokukaihei 6-118406) and 6-194645/1994 (Tokukaihei 6-194645) disclose techniques for combining the foregoing pixel division method with use of an optical retardation plate.
The liquid crystal display device disclosed in Japanese Unexamined Patent Publication No. 6-118406/1994 aims for improvement of contrast, etc. by providing an optical anisotropic film (optical retardation plate) between a liquid crystal panel and a polarizing plate. Again, a compensation plate (optical retardation plate) disclosed in Japanese Unexamined Patent Publication No. 6-194645/1994 is set so as to have almost no in-plane refractive index (in a direction parallel to the surface of the compensation plate), and so that the refractive index in a direction perpendicular to the surface of the compensation plate is smaller than the in-plane refractive index. Accordingly, this compensation plate has a negative refractive index. For this reason, the positive refractive index arising in the liquid crystal display element when a voltage is applied can be compensated, and viewing angle dependence reduced.
However, today, when liquid crystal display devices with a wider viewing angle field and better display quality are called for, further improvement of viewing angle dependence is needed. Accordingly, use of an optical retardation plate with an inclined index ellipsoid, like that in the foregoing U.S. Pat. No. 5,506,706 (Japanese Unexamined Patent Publication No. 6-75116/1994), is not always sufficient, and there is still room for improvement.
Further, although the foregoing pixel division method for resolving reversal phenomenon is admittedly able to do so when the viewing angle is inclined up or down, at that time, contrast is impaired, and black display takes on a white cast, thus appearing gray. A further drawback is that viewing angle dependence arises when the viewing angle is inclined to the left or right.
Again, with the foregoing method for combining pixel division with use of an optical retardation plate, when viewing angle is inclined, coloring phenomenon occurs at an incline of 45xc2x0. Further, since the foregoing method uses a liquid crystal display element in which each pixel is divided into two divisions of equal area, there is limited suppression of impairment of contrast when the viewing angle is inclined up or down. This is due to the following reason.
With the foregoing pixel division method, since each pixel is divided into two divisions of equal area, viewing angle characteristics in the standard viewing angle direction (the direction in which, when viewed from a direction perpendicular to the screen, display contrast improves) and in the opposite viewing angle direction (the direction in which, when viewed from a direction perpendicular to the screen, display contrast worsens) are equalized. However, since actual viewing angle characteristics in the standard viewing angle direction are opposite from those in the opposite viewing angle direction, even if use of an optical retardation plate is combined with pixel division, it is difficult to uniformly suppress impairment of contrast when viewing angle is inclined up or down. When viewing angle is inclined in the standard viewing angle direction, in particular, reversal phenomenon and blacking out of the screen tend to occur.
The object of the present invention is to further improve viewing angle dependence beyond the compensation effect of an optical retardation plate having an inclined index ellipsoid, and, in particular, to minimize reversal phenomenon occurring when viewing angle is inclined in the upper or lower direction, to uniformly suppress the impairment of contrast and the tendency for a displayed image to appear in white which occur in such a situation, and to effectively improve coloring phenomenon.
In order to attain the foregoing object, a liquid crystal display device according to the present invention is made up of a liquid crystal display element which includes a pair of transparent substrates provided, on respective facing surfaces thereof, with transparent electrodes and alignment layers, and a liquid crystal layer filling a space between the transparent substrates; a pair of polarizers, one provided on each side of the liquid crystal display element; and at least one optical retardation plate, provided between the liquid crystal display element and one of the polarizers, which has an inclined index ellipsoid; in which the alignment films align the liquid crystal in a different direction in each of a plurality of liquid crystal layer divisions of unequal area into which each pixel is divided, and respective extents of variation in response to wavelength of light of (i) anisotropy of the refractive index of a liquid crystal material of the liquid crystal layer and (ii) anisotropy of the refractive index of the optical retardation plate(s) are set to within a range such that viewing-angle-dependent coloring of the liquid crystal screen does not occur.
With the foregoing structure, when linearly polarized light passes through the liquid crystal layer having birefringence, ordinary light and extraordinary light are produced, thus converting the linearly polarized light into elliptically polarized light, but the linearly polarized light is compensated by the optical retardation plate having an inclined index ellipsoid.
However, when further improvement of viewing angle dependence is called for, it is not necessarily sufficient to rely solely on the foregoing compensation effect. Accordingly, as a result of further research, the inventors found that viewing-angle-dependent coloring of the liquid crystal screen is influenced by the respective extents of variation in response to wavelength of light of (i) anisotropy of the refractive index of the liquid crystal material of the liquid crystal layer and (ii) anisotropy of the refractive index of the optical retardation plate.
For this reason, in the liquid crystal display device having the present structure, the respective extents of variation in response to wavelength of light of (i) anisotropy of the refractive index of the liquid crystal material of the liquid crystal layer and (ii) anisotropy of the refractive index of the optical retardation plate are set to within a range such that viewing-angle-dependent coloring of the liquid crystal screen does not occur.
By this means, in the foregoing liquid crystal display device, coloring of the screen can be better prevented, and contrast fluctuation and reversal phenomenon can also be improved beyond the compensation effect of the optical retardation plate alone.
Further, the liquid crystal display device according to the present structure uses a divided liquid crystal layer, in which the liquid crystal layer in each pixel is divided into two divisions of unequal area, in each of which the liquid crystal molecules are aligned in a different direction.
By this means, in the foregoing liquid crystal display device, the difference between the opposite viewing angle characteristics in the standard and opposite viewing angle directions can be eliminated, and viewing angle characteristics in both viewing angle directions can be brought into closer conformity. Consequently, in the foregoing liquid crystal display device, impairment of contrast and the tendency for the displayed image to appear in white, which occur when the viewing angle is inclined upward or downward, can be suppressed nearly uniformly, and black, especially, can be displayed more clearly.
Further, in order to attain the foregoing object, a liquid crystal display device according to a second structure of the present invention is made up of a liquid crystal display element, which includes a pair of transparent substrates, provided with, on respective facing surfaces thereof, transparent electrodes and alignment films, and a layer of liquid crystal, aligned with a 90xc2x0 twist, filling a space between the transparent substrates; a pair of polarizers, one provided on each side of the liquid crystal display element; and at least one optical retardation plate, provided between the liquid crystal display element and one of the polarizers, having an index ellipsoid whose three principal refractive indices na, nb, and nchave a relation na=nc greater than nb, the index ellipsoid inclining because the directions of (a) the principal refractive index nb, which is parallel to the normal direction of the surface of the optical retardation plate, and (b) the principal refractive index nc or na, which is in-plane with respect to the surface of the optical retardation plate, incline in a clockwise or counter-clockwise direction with respect to an axis which is the direction of the refractive index na or nc; in which the alignment films align the liquid crystal in a different direction in each of a plurality of liquid crystal layer divisions of unequal area into which each pixel is divided, and respective extents of variation in response to wavelength of light of (i) anisotropy of the refractive index of a liquid crystal material of the liquid crystal layer and (ii) anisotropy of the refractive index of the optical retardation plate(s) are set to within a range such that viewing-angle-dependent coloring of the liquid crystal screen does not occur.
With the foregoing structure, when linearly polarized light passes through the liquid crystal layer having birefringence, ordinary light and extraordinary light are produced, thus converting the linearly polarized light into elliptically polarized light, but the linearly polarized light is compensated by the optical retardation plate, which has principal refractive indices na, nb, and nc fulfilling the relation na=nc greater than nb, and an index ellipsoid whose short axis, which includes the principal refractive index nb, inclines with respect to the normal direction of the optical retardation plate.
However, when further improvement of viewing angle dependence is called for, it is not necessarily sufficient to rely solely on the foregoing compensation effect. Accordingly, as a result of further research, the inventors found that viewing-angle-dependent coloring of the liquid crystal screen is influenced by the respective extents of variation in response to wavelength of light of (i) anisotropy of the refractive index of the liquid crystal material of the liquid crystal layer and (ii) anisotropy of the refractive index of the optical retardation plate.
For this reason, in the liquid crystal display device having the present structure, the respective extents of variation in response to wavelength of light of (i) anisotropy of the refractive index of the liquid crystal material of the liquid crystal layer and (ii) anisotropy of the refractive index of the optical retardation plate are set to within a range such that viewing-angle-dependent coloring of the liquid crystal screen does not occur.
By this means, in the foregoing liquid crystal display device, coloring of the screen can be better prevented, and contrast fluctuation and reversal phenomenon can also be improved beyond the compensation effect of the optical retardation plate alone.
Further, the liquid crystal display device according to the present structure uses a divided liquid crystal layer, in which the liquid crystal layer in each pixel is divided into two divisions of unequal area, in each of which the liquid crystal molecules are aligned in a different direction.
By this means, in the foregoing liquid crystal display device, the difference between the opposite viewing angle characteristics in the standard and opposite viewing angle directions can be eliminated, and viewing angle characteristics in both viewing angle directions can be brought into closer conformity. Consequently, in the foregoing liquid crystal display device, impairment of contrast and the tendency for the displayed image to appear in white, which occur when the viewing angle is inclined upward or downward, can be suppressed nearly uniformly, and black, especially, can be displayed more clearly.
The following will concretely specify, in the liquid crystal display devices having the first and second structures above, the range of the extents of variation within which viewing-angle-dependent coloring of the liquid crystal screen does not occur. Namely, a ratio xcex94nL(450)/xcex94nL(550), which is a ratio of xcex94nL(450), anisotropy of the refractive index of the liquid crystal material of the liquid crystal layer in response to light of 450 nm wavelength, to xcex94nL(550), anisotropy of the refractive index of the liquid crystal material in response to light of 550 nm wavelength; and a ratio xcex94nF(450)/xcex94nF(550), which is a ratio of xcex94nF(450), anisotropy of the refractive index of the optical retardation plate(s) in response to light of 450 nm wavelength, to xcex94nF(550), anisotropy of the refractive index of the optical retardation plate(s) in response to light of 550 nm wavelength, are set so as to satisfy:   0  ≤                    (                  Δ          ⁢                      xe2x80x83                    ⁢                                                    n                L                            ⁡                              (                450                )                                      /            Δ                    ⁢                      xe2x80x83                    ⁢                                    n              L                        ⁡                          (              550              )                                      )            -      1                      (                  Δ          ⁢                      xe2x80x83                    ⁢                                                    n                F                            ⁡                              (                450                )                                      /            Δ                    ⁢                      xe2x80x83                    ⁢                                    n              F                        ⁡                          (              550              )                                      )            -      1         less than   0.25
Alternatively, a ratio xcex94nL(650)/xcex94nL(550), which is a ratio of xcex94nL(650), anisotropy of the refractive index of the liquid crystal material of the liquid crystal layer in response to light of 650 nm wavelength, to xcex94nL(550), anisotropy of the refractive index of the liquid crystal material in response to light of 550 nm wavelength; and a ratio xcex94nF(650)/xcex94nF(550), which is a ratio of xcex94nF(650), anisotropy of the refractive index of the optical retardation plate(s) in response to light of 650 nm wavelength, to xcex94nF(550), anisotropy of the refractive index of the optical retardation plate(s) in response to light of 550 nm wavelength, are set so as to satisfy:   0  ≤                    (                  Δ          ⁢                      xe2x80x83                    ⁢                                                    n                L                            ⁡                              (                650                )                                      /            Δ                    ⁢                      xe2x80x83                    ⁢                                    n              L                        ⁡                          (              550              )                                      )            -      1                      (                  Δ          ⁢                      xe2x80x83                    ⁢                                                    n                F                            ⁡                              (                650                )                                      /            Δ                    ⁢                      xe2x80x83                    ⁢                                    n              F                        ⁡                          (              550              )                                      )            -      1         less than   0.25
If at least one of the foregoing ranges is used, the liquid crystal display device according to the present invention will show slight coloring at a viewing angle of 50xc2x0, which is the viewing angle field required of ordinary liquid crystal display devices, but will be satisfactory for use when viewed from whatever direction.
In a liquid crystal display device with a wider viewing angle field of 70xc2x0, it is preferable to set the range of the extents of variation so as to satisfy:   0  ≤                    (                  Δ          ⁢                      xe2x80x83                    ⁢                                                    n                L                            ⁢                              (                450                )                                      /            Δ                    ⁢                      xe2x80x83                    ⁢                                    n              L                        ⁢                          (              550              )                                      )            -      1                      (                  Δ          ⁢                      xe2x80x83                    ⁢                                                    n                F                            ⁢                              (                450                )                                      /            Δ                    ⁢                      xe2x80x83                    ⁢                                    n              F                        ⁢                          (              550              )                                      )            -      1        ≤  0.17
or so as to satisfy:   0  ≤                    (                  Δ          ⁢                      xe2x80x83                    ⁢                                                    n                L                            ⁢                              (                650                )                                      /            Δ                    ⁢                      xe2x80x83                    ⁢                                    n              L                        ⁢                          (              550              )                                      )            -      1                      (                  Δ          ⁢                      xe2x80x83                    ⁢                                                    n                F                            ⁢                              (                650                )                                      /            Δ                    ⁢                      xe2x80x83                    ⁢                                    n              F                        ⁢                          (              550              )                                      )            -      1        ≤  0.18
By using at least one of the foregoing ranges, the liquid crystal display device according to the present invention will show no coloring at a viewing angle of 70xc2x0, which is the viewing angle field required of wide viewing angle field liquid crystal display devices, when viewed from whatever direction.
Further, in the liquid crystal display devices having the first and second structures above, it is preferable to set xcex94nL(550), which is the anisotropy of the refractive index of the liquid crystal material of the liquid crystal layer in response to light of 550 nm wavelength, within a range of more than 0.060 and less than 0.120.
This is because, depending on the viewing angle direction, reversal phenomenon, impairment of the contrast ratio, etc. were found to occur when xcex94nL(550), the anisotropy of the refractive index of the liquid crystal material in response to light of 550 nm wavelength (the central range of the visible light range) was set to 0.060 or less, or to 0.120 or more. Accordingly, in the foregoing liquid crystal display devices, by setting xcex94nL(550), the anisotropy of the refractive index of the liquid crystal material in response to light of 550 nm wavelength, within a range of more than 0.060 and less than 0.120, phase differences corresponding to viewing angle which arise in the liquid crystal display element can be resolved, and thus not only coloring phenomenon occurring in the liquid crystal screen depending on viewing angle, but also contrast fluctuation, reversal phenomenon in the right and left viewing angle directions, etc. can be further improved.
In this case, in the foregoing liquid crystal display devices, it is more preferable to set xcex94nL(550), which is the anisotropy of the refractive index of the liquid crystal material of the liquid crystal layer in response to light of 550 nm wavelength, within a range of more than 0.070 and less than 0.095.
By this means, in the foregoing liquid crystal display devices, phase differences corresponding to viewing angle which arise in the liquid crystal display element can be resolved more effectively, and thus contrast fluctuations, reversal phenomenon in the right and left directions, and coloring phenomenon can be improved with certainty.
Further, in the liquid crystal display devices having the first and second structures above, it is preferable to set the angle of inclination of the index ellipsoid of each optical retardation plate within a range of 15xc2x0 through 75xc2x0.
By setting the angle of inclination of the index ellipsoid of each optical retardation plate provided in the liquid crystal display device within a range of 15xc2x0 through 75xc2x0, the aforementioned effect of the optical retardation plate, of compensating the phase difference, can be obtained with certainty.
Further, in the liquid crystal display devices having the first and second structures above, it is preferable if, in each optical retardation plate, (naxe2x88x92nb)xc3x97d, which is a product of a difference between the principal refractive indices na and nb and the thickness d of the optical retardation plate, is set within a range from 80 nm through 250 nm.
By setting (naxe2x88x92nb)xc3x97d, which is a product of a difference between the principal refractive indices na and nb and the thickness d of the optical retardation plate, within a range from 80 nm through 250 nm for each optical retardation plate provided in the liquid crystal display device, the aforementioned effect of the optical retardation plate, of compensating the phase difference, can be obtained with certainty.
Further, in the liquid crystal display devices having the first and second structures above, it is preferable if each optical retardation plate is provided such that, in the largest of the liquid crystal layer divisions in a given pixel, the direction in which the liquid crystal molecules in the vicinity of the inner surface of the alignment film incline when a voltage is applied by the transparent electrodes is opposite the direction of incline of the index ellipsoid.
In the foregoing structure, if, in the largest of the liquid crystal layer divisions in a given pixel, the direction of incline of the index ellipsoid of the optical retardation plate is opposite to the direction of incline of the liquid crystal molecules when a voltage is applied, the optical characteristics due to the liquid crystal molecules can be set opposite to the optical characteristics due to the index ellipsoid, i.e., due to the optical retardation plate. Accordingly, in the foregoing liquid crystal display devices, the optical retardation plate can compensate the bias in optical characteristics which arises since the liquid crystal molecules near the inner surface of the alignment film do not tilt up even when a voltage is applied.
By this means, reversal phenomenon when the viewing angle is inclined in the standard viewing angle direction can be suppressed, and a good display image, which will not black out, can be obtained. Further, impairment in contrast when the viewing angle is inclined in the opposite viewing angle direction can be suppressed, and thus a good display image, which will not appear in white, can be obtained. Moreover, reversal phenomenon in the right and left viewing angle directions can be suppressed.
Further, in the liquid crystal display devices having the first and second structures above, when the liquid crystal layer divisions are a first liquid crystal layer division and a second liquid crystal layer division, it is preferable if the ratio in size of the first liquid crystal layer division to the second liquid crystal layer division is set within a range from 6:4 through 19:1.
By this means, in a liquid crystal display device having the foregoing structure with an index ellipsoid with a specific direction of inclination, viewing angle characteristics can be further improved.
In this case, by setting the ratio in size of the first liquid crystal layer division to the second liquid crystal layer division within a range from 7:3 through 9:1, viewing angle characteristics of the foregoing liquid crystal display devices can be improved so as to obtain good viewing angle characteristics which are balanced in the upper and lower viewing angle directions.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.